1. Field of the Invention
This invention relates generally to leak detectors and more particularly to a leak detector that automatically compensates for gain variations, component aging, drift characteristics of associated circuits as well as other non-desirable variables without requiring operator intervention.
2. Description of the Prior Art
One of the most common methods used to detect refrigerant gas leaks is by producing an electrical discharge or corona between two electrodes and sensing the change in corona current when refrigerant gas is present. There are many problems or variables which must be considered when designing a practical corona discharge detector. Some of these apply only to the corona gap sensor itself while others apply to both the sensor and the associated circuitry used to detect the current changes and produce an output. The following is an outline of some of the factors which must be considered when designing such an instrument.
I. Modes of "Drift" in Corona Sensor and High Voltage Supply
1. Temperature PA0 2. Humidity PA0 3. Aging PA0 4. Power supply changes due to temperature effects and in battery-powered units, the battery condition PA0 1. Mechanical Construction PA0 2. Materials PA0 1. Temperature effects of components and thresholds PA0 2. Leakage paths PA0 1. Component Tolerances PA0 2. Variations in threshold values of active components PA0 3. Variations in gain of active components PA0 Lieberman U.S. Pat. No. 4,282,521; PA0 Roberts U.S. Pat. No. 3,076,139; PA0 Jeffers, Sacerio U.S. Pat. No. 4,488,118;
II. Other "Variables" in Corona Sensor
III. Modes of "Drift" in Detecting an Alarm Circuit
IV. Other "Variables" in Detecting an Alarm Circuit
Many methods have been proposed to compensate for some of these variables in a sensor for detecting refrigerant gas, which are very similar to and are present in almost all types of sensors to some degree. The following is a list of known patents which describe various means of adjusting and/or compensating for some of these variables:
Lieberman U.S. Pat. No. 4,282,521 is described as a method and apparatus for automatically recalibrating a leak detector. However, this circuit is nothing more than a circuit designed to balance at the level seen by the sensor at the time the operator turns the unit on. The circuit does this by storing a charge on a capacitor representative of the signal at turn-on, and subsequently sounding an alarm only if the signal at the input exceeds that stored value. It does nothing automatically and requires operator intervention to re-initiate the balance cycle. Theoretically, by turning the circuit on one in a contaminated area, the circuit will respond only to a higher concentration than that seen at turn on.
Another method commonly used to adjust the sensitivity is by adjusting a potentiometer. This method has the advantage over the Lieberman patent by knowing approximately how much you are desensitizing the unit and enables you to tune out only that portion that you desire. However, this manual method is time-consuming and requires some degree of operator skill.
Another technique is to compacitively couple the sensor signal to an alarm circuit which is designed to give a broad response versus freon concentration eliminating the need to bias out unwanted signals or re-adjust the sensitivity at all. However, since the alarm circuit is always active, it can be annoying to the operator when heavy contamination exists.
Another method is described in Jeffers, Sacerio U.S. Pat. No. 4,488,118 which adjusts the corona voltage to produce a predetermined corona current regardless of the background contamination level. However, this method also requires operator intervention to reset or rebalance the instrument. In addition, U.S. Pat. No. 4,831,332 describes a gas leak detector.
Therefore, a need exists for a leak detector which overcomes all of the disadvantages of the prior art including operator intervention, gain variations, component sensitivities and drift characteristics of components.